1. Field of the Invention
The present invention relates to a polarization separation device used to illuminate a spatial light modulation device such as a liquid crystal panel that utilizes polarization of light. The present invention relates also to a projection-type display apparatus having such a polarization separation device.
2. Description of the Prior Art
Conventionally, as projection-type display apparatuses that project an enlarged image of an original image through a projection lens are known those employing a CRT and those employing a light source and a spatial light modulation device. Here, as spatial light modulation devices are known transmission-type liquid crystal panels that use twisted nematic liquid crystal. Liquid crystal panels of this type are in practical use in various applications because they permit miniaturization of projection-type display apparatuses, because they permit projection of high-resolution images simply if provided with sufficient numbers of pixels, and because their mass-production methods have been well established with those manufactured for direct-view purposes.
A spatial light modulation device such as one using twisted nematic liquid crystal utilizes polarization of light, and therefore has polarizers provided at its entrance and exit sides. Out of the light that illuminates the spatial light modulation device, the linearly polarized light component that has passed through the entrance-side polarizer then has its polarization state modulated spatially while passing inside the spatial light modulation device. This controls the amount of light that passes through the exit-side polarizer, and thereby forms an optical image.
A projection-type display apparatus employing a spatial light modulation device typically uses a lamp that emits natural light to illuminate the spatial light modulation device. If the spatial light modulation device is of a type that utilizes polarization of light, the polarizer provided at its entrance side transmits only about one half of the natural light emitted from the lamp, and the other half of the light is wasted by being reflected or absorbed.
To overcome this inconvenience, various techniques have been proposed that are generally called polarization conversion. According to these techniques, the natural light from a light source is separated beforehand into, on the one hand, a polarized light component (hereafter referred to as the primary polarized light component) polarized in the same way as the light that a spatial light modulation device is designed to use and, on the other hand, a polarized light component (hereafter referred to as the secondary polarized light component) polarized perpendicularly thereto. Then, the polarization plane of the secondary polarized light component, which if left intact the spatial light modulation device cannot use, is rotated through 90.degree. so that the primary and secondary polarized light components are, after their polarization planes are thus made identical, together fed to the spatial light modulation device. In this way, it is possible to use both of the two polarized light components.
Accordingly, a projection-type display apparatus utilizing polarization conversion needs to be provided with a polarization separation device for separating natural light into two polarized light components polarized in directions perpendicular to each other and a polarization plane rotating device for rotating the polarization plane of one of those two separated polarized light components through 90.degree.. As polarization separation devices, polarization separation multilayer films are widely known that utilize the Brewster angle and interference and that are available in plate-shaped and prism-shaped types.
On the other hand, as polarization plane rotating devices, phase films called .lambda./2 plates are generally known. A .lambda./2 plate is made by drawing an optically transparent organic film uniaxially so as to give it optical anisotropy. It has its thickness and optical anisotropy so controlled as to give the light passing therethrough a phase difference that corresponds to one half of the wavelength of the light. Accordingly, if linearly polarized light having a polarization plane in a direction 45.degree. with respect to an optical axis enters a .lambda./2 plate, it exits therefrom as linearly polarized light having a polarization plane rotated further through 90.degree..
A projection-type display apparatus having a polarization separation device and a polarization plane rotating device as described above is disclosed in Japanese Laid-Open Patent Application H6-202094. The construction of this projection-type display apparatus is shown in FIG. 14. The natural light radiated from a light source 901 is made into a parallel beam by a parabolic surface mirror 902, and then enters a polarization separation device 903. The primary and secondary polarized light components exiting from the polarization separation device 903 travel through a first and a second lens array 904 and 905, and then illuminate a liquid crystal panel 907.
The first lens array 904 separates the beam of the illumination light into partial beams, and the thus separated partial beams are enlarged by the second lens array 905 to an appropriate size. The separated partial beams are then superimposed on each other on the liquid crystal panel 907 by a convex lens 908. Another convex lens 909 provided in the vicinity of the liquid crystal panel 907 makes the principal ray within each angle of view parallel to the optical axis.
The polarization separation device 903 has a structure as shown in FIG. 15. A structure composed by putting together a prism having an isotropic refractive index and a prism layer made of a birefringent material in general is widely known as a Wollaston prism. This structure exhibits, at the interface between the prism and the prism layer, different refraction conditions in different polarization directions perpendicular to each other, and thereby permits two polarized light components to travel in different directions.
The polarization separation device 903 has a plurality of such Wollaston prisms arranged in an array. Thus, the polarization separation device 903 is composed of a prism array base plate 911 having a blaze-shaped section, a flat base plate 912, and a birefringent optical material layer 913 made of an optically anisotropic material. Here, since calcite, which is generally used as an optically anisotropic material, is expensive, a material produced by uniaxially arranging strips of an organic material such as liquid crystal layers, organic films, or monomers is used.
Thus, the polarization separation device 903 separates the light 914 entering it into a primary polarized light component 914a and a secondary polarized light component 914b that exit therefrom traveling in directions .theta.' degrees apart from each other. As a result, the light beams that the first lens array 904 makes converge on the second lens array 905 each form separate spots, a predetermined distance apart from each other in the direction of the angle .theta.', for the primary and secondary polarized light components 914a and 914b.
In the vicinity of the second lens array 905, a phase difference plate 906 is provided that selectively acts on the spots formed by the secondary polarized light component so as to rotate its polarization plane through 90.degree.. As a result, the polarization planes of the primary and secondary polarized light components exiting from the convex lens 908 are made uniform. By aligning the polarization plane of these polarized light components with the polarization direction of the entrance-side polarizer (not shown) of the liquid crystal panel 907, it is possible to realize an optical system that permits efficient use of light.
The prism array base plate 911 used in this polarization separation device 903 utilizes diffraction of light, and thus has prisms arranged with a pitch P of about 1 mm. This requires a considerably thick birefringent optical material layer 913. However, it is generally difficult to form a thick birefringent optical material layer 913 by uniaxially arranging strips of an optically anisotropic material. For this reason, a polarization separation device in which a diffraction grating is used instead of a prism array base plate 911 is disclosed in Japanese Laid-Open Patent Application H10-197827.
This polarization separation device is shown in FIG. 16. The polarization separation device 101 is composed of a diffraction gating 102 having a blaze-shaped section and having a pitch D and a height H both of the order of several micrometers and a birefringent optical material layer 103 made of an optically anisotropic material. As an optically anisotropic material having birefringence, a material produced by uniaxially orienting acicular liquid crystal molecules is used.
According to known methods, orientation is achieved, for example, by forming an organic orientation film of polyimide or the like on a base plate and then rubbing the film with a rubbing cloth in one direction so as to form fine grooves (this method will hereafter be referred to as "rubbing"), or by vapor-depositing SiO.sub.2 obliquely on a base plate so as to form fine grooves, or by ion-milling a base plate so as to form fine grooves (this method will hereafter be referred to as "grating"). All of these methods cause a force that tends to align the length direction of the liquid crystal molecules with the grooves, and thus the liquid crystal molecules are oriented uniaxially.
If the refractive index of the optically anisotropic material for a primary polarized light component is made substantially equal to the refractive index of the diffraction grating and the refractive index of the optically anisotropic material for a secondary polarized light component is made different from the refractive index of the diffraction grating, when the first and secondary polarized light components pass through the polarization separation device 101, the former is allowed to travel straight and the latter is diffracted. Thus, light 104 entering the polarization separation device 101 is separated into a primary polarized light component 104a and a secondary polarized light component 104b that exit therefrom traveling in directions apart from each other by the angle of diffraction .theta.. This helps make the birefringent optical material layer 103 thin and easy to produce.
However, in this polarization separation device 101 disclosed in H10-197827, as shown in FIG. 17, the diffraction grating 102 having a blaze-shaped section has both effective surfaces 102a that are involved in diffraction and non-effective surfaces 102b that has nothing to with diffraction.
The diffraction grating 102 is formed by molding using a metal mold. Therefore, to secure allowances for manufacturing errors in the metal mold and to secure drafts for easing the removal of the metal mold in the molding process, the non-effective surfaces 102b are formed at an angle .alpha. relative to the direction perpendicular to the entrance surface 101a of the polarization separation device 101. As a result, as indicated by hatching in the figure, part of the incident light 104 strikes the non-effective surfaces 102b, and this lowers the diffraction efficiency with which the light is diffracted in the desired direction.
Moreover, in cases where the liquid crystal molecules are oriented uniaxially by rubbing, since the diffraction grating 102 that serves as a base plate has a blaze-shaped section, the rubbing cloth does not reach to the bottom portions thereof. This makes satisfactory orientation of the liquid crystal molecules impossible. On the other hand, orientation by vapor-depositing of SiO.sub.2 or by grating requires not only extra production steps but also expensive equipment such as a vapor-depositing, ion-milling, or other apparatus, and thus raises the manufacturing costs.
Moreover, a polarization conversion optical system requires the provision of a first and a second lens array 904 and 905 and a polarization separation device 903 or 101; that is, it requires a comparatively large number of components, and thus raises the costs of the projection-type display apparatus as a whole that incorporates it.